1. Field of the Invention
The present invention relates to a method and system for acquiring flow velocities in an ultrasound diagnosis apparatus.
2. Description of the Related Art
In an ultrasound diagnosis method of transmitting an ultrasound beam to a living body, and receiving waves reflected from tissues of the living body to acquire information of the living body, information associated with soft tissues can be acquired without radiation hazards caused by X-rays and without the use of a contrast medium.
An array type piezoelectric transducer is used for an ultrasound probe in a recent ultrasound diagnosis apparatus. Ultrasound beams generated by transducers of the ultrasound probe are transmitted to a living body. Since an echo signal received from the living body by the transducers is delayed by a predetermined time period, the ultrasound beams are focused on a predetermined determined position in the living body, and hence directional resolution is improved. Therefore, a high-resolution ultrasound tomographic image can be acquired.
In a linear electronic scanning type ultrasound blood flow measurement apparatus shown in FIGS. 1A and 1B, a repeating pulse signal for determining a transmission interval of the ultrasound beam is input from a pulse generator 8 to transmission delay units 2b-l to 2b-n. In the transmission delay units 2b-l to 2b-n, the repeating pulse signals delayed by the predetermined delay times determined on the basis of a transmitting direction and focal point of the ultrasound beams are input to pulsers 2a-l to 2a-n for respectively generating drive pulse signals for driving transducers 1-l to 1-n of an ultrasound probe 1. When the drive pulse signals are respectively supplied to the transducers 1-l to 1-n by a transducer selecting switch 7, the ultrasound beams are transmitted to the living body.
On the other hand, the ultrasound beams reflected from the living body are received by the transducers 1-l to 1-n as echo signals. The received echo signals are respectively input to preamplifiers 3a-l to 3a-n through the transducer selecting switch 7, and are amplified to have predetermined amplitudes. In reception delay units 3b-l to 3b-n, the echo signals are delayed by delay times substantially equal to the delay times in the transmission delay units 2b-l to 2b-n. In an adder 9, addition of the echo signals delayed by the predetermined delay times is performed. The added echo signal is input to B- and D-mode processing units 4 and 5.
In a logarithm amplifier 4a in the B-mode processing unit 4, logarithm conversion corresponding to the amplitude of the echo signal is performed. In an envelope detector 4b, an envelope of the echo signal is detected, and is A/D-converted by an A/D (analog/digital) converter 4c. Thereafter, the converted signal is stored in an image memory 6a of a display unit 6. An ultrasound tomographic image is displayed on a TV monitor 6b.
In the D-mode processing unit 5, a reference signal having a frequency substantially equal to that of the echo signal is input from a reference signal generator 10b to a phase detector 5b. In addition, a shift signal obtained by phase-shifting the reference signal by 90.degree. by a phase shifter 10a is input to a phase detector 5a. In the phase detectors 5a and 5b, the phase of the echo signal output from the adder 9 is detected on the basis of the shift and reference signals. The echo signals having phases different from each other by 90.degree. are input to MTI (moving target indicator) filters 5g and 5h through low pass filters 5c and 5d, and A/D converters 5e and 5f, respectively.
When a Doppler signal is acquired, a phase shift amount (Doppler shift amount) within a unit time period of the echo signal is acquired by scanning the same portion at a predetermined interval. For example, a predetermined number of scanning operations are performed with respect to the same portion, and further a blood flow velocity at a predetermined depth is calculated on the basis of the echo signals obtained by performing a predetermined number of scanning operations with respect to the same portions. Note that each echo signal includes not only echo signals (Doppler signals) reflected from a moving object such as blood corpuscles, but also echo signals from a fixed reflector.
In order to eliminate the echo signals (clutter signals) from the fixed reflector, a predetermined number of echo signals at a predetermined depth are input to the MTI filters 5g and 5h. Note that the MTI technique is generally known in the field of radars. The clutter signals are eliminated by the MTI filters 5g and 5h, and the echo signals (Doppler signals) from only the blood corpuscles are input to an operation unit 5i. In the operation unit 5i, a frequency analysis is performed on the basis of the echo signals from which the clutter signals are eliminated, and the center or dispersion of the spectrum is calculated. The calculated value is stored in the image memory 6a. A tomographic image and a blood flow image (Doppler image) are displayed on the TV monitor 6b.
When a blood flow velocity of a predetermined portion is observed, it is generally known that the larger the number of echo signals reflected from the same portion is, the higher the precision of measurement. In particular, when a clutter signal must be sufficiently eliminated (e.g., when the clutter signal is extremely large or the frequency of the Doppler signal is close to the frequency of the clutter signal), a large number of echo signals must be acquired. The acquisition time period of the Doppler image is, therefore, longer than that of the B-mode image. For this reason, in a sector scanning operation, a parallel simultaneous reception method is used as a method of executing real time processing.
In the parallel simultaneous reception method in the sector scanning operation, as shown in FIG. 2, two receiving directions b1 and b2 are set with respect to a transmitting direction a of the ultrasound beam from the ultrasound probe 1. Note that two reception circuits having a reception directivity with respect to the receiving directions b1 and b2 are used. For example, an ultrasound beam having a relatively large beam width is transmitted along the transmitting direction a and the echo signals are simultaneously received along the receiving directions b1 and b2 shifted from the transmitting direction a by .+-..DELTA..theta.. By this method, scanning operations in the two directions adjacent to each other by .DELTA..theta. are simultaneously performed. Therefore, an image acquisition time period can be half that in the conventional method.
For example, however, in color display of a blood flow image, color processing is not performed for a signal having no predetermined amplitude such as a noise signal. On the other hand, if a normal echo signal does not have a predetermined amplitude due to an influence of a speckle, color processing associated with the echo signal is not performed, and hence an image quality is undesirably degraded.
Thus, a demand has arisen for developing an ultrasound diagnosis apparatus which can display a blood flow image in real time without degradation of an image quality due to a speckle.